Re: [virtio-dev] Re: [PATCH] virtio-balloon: Disable free page hinting/reporting if page poison is disabled

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Apr 17, 2020 at 12:26:24PM +0200, David Hildenbrand wrote:
> On 17.04.20 12:19, Michael S. Tsirkin wrote:
> > On Fri, Apr 17, 2020 at 12:09:38PM +0200, David Hildenbrand wrote:
> >>  > What do you call "hinting ends" though? The fact we put
> >>> a page in the VQ is not a guarantee that it's been consumed
> >>> by the hypervisor.
> >>>
> >>
> >> I'd say hinting ends once the hypervisor sets FREE_PAGE_REPORT_S_DONE.
> > 
> > Can't find that one anywhere. what did I miss?
> 
> Sorry, the QEMU implementation is confusing. FREE_PAGE_REPORT_S_DONE is
> translated to VIRTIO_BALLOON_CMD_ID_DONE

Well VIRTIO_BALLOON_CMD_ID_DONE just means "don't give me any
more hints, I finished migration".
Guest will stop hinting even without that once it scans all
free memory.



> QEMU:
> 
> hw/virtio/virtio-balloon.c:virtio_balloon_free_page_report_notify()
> -> virtio_balloon_free_page_done(dev)
> -> s->free_page_report_status = FREE_PAGE_REPORT_S_DONE;
>    virtio_notify_config(vdev);
> 
> When the guest reads the config
> hw/virtio/virtio-balloon.c:virtio_balloon_get_config()
> -> if (dev->free_page_report_status == FREE_PAGE_REPORT_S_DONE)
> -> config.free_page_report_cmd_id = ... VIRTIO_BALLOON_CMD_ID_DONE
> 
> 
> Linux:
> 
> drivers/virtio/virtio_balloon.c:report_free_page_func()
> -> if (cmd_id_received == VIRTIO_BALLOON_CMD_ID_DONE) {
> -> return_free_pages_to_mm()
> 
> 
> So it's VIRTIO_BALLOON_CMD_ID_DONE.
> 
> 
> -- 
> Thanks,
> 
> David / dhildenb